1. Field of the Invention
This invention relates to a method and apparatus for improving the thermal performance of a cooking utensil, and more particularly to a cooking utensil which may be selectively controlled to vary the thermal conductance and insulating characteristics thereof.
2. Description of the Prior Art
Cookware for domestic and commercial food preparation has been relatively unchanged for the past thirty years. Nevertheless, food preparation personnel continue to require cooking utensils which prevent or reduce the chances of overcooking, scorching, or burning food by evenly distributing the heat across the entire cooking surface of the utensil and by thermostatically controlling the cooking temperature thereof. In addition, cooking utensils and/or food-serving containers with an improved ability to retain heat are required.
To evenly distribute the heat, cookware manufacturers have experimented with different cookware materials including ceramics and metals such as stainless steel, aluminum, and copper. More particularly, laminates with combinations of these materials have been used. In addition, double-boilers have been developed which include both an inner and an outer cooking vessel, the inner vessel being surrounded by a bath of boiling water or steam within the outer cooking vessel. Double-boilers suffer from the disadvantages of eventual boil-off of all of the water within the outer vessel, thereby losing the thermal constraint as well as the eventual build up of mineral deposits from the evaporated water. Further, double-boilers have excessive weight and maximum cooking temperatures in the inner vessel which are only as high as the boiling temperature of the water. Similarly, U.S. Pat. No. 1,493,459 issued to Jancikin discloses a cooking utensil similar to a double-boiler into which water or oil may be placed into the outer cooking vessel.
U.S. Pat. No. 1,555,484 issued to Rosenberg discloses a cooking utensil with a layered arrangement including a heat-receiving plate below a heat-distributing plate which, in turn, is below a cooking surface of the utensil. The heat-distributing plate is of a relatively high conductivity material such as copper which may be corrugated or formed with a series of air cells.
U.S. Pat. No. 1,961,643 issued to Roth discloses a cooking plate having a pair of parallel upper and lower surfaces. The lower surface is exposed to a source of heat and the upper surface receives a cooking utensil. Located between the upper and lower plates is a filling of gravel, silica, sand, cement, soapstone, crushed lava or other siliceous material which distributes the heat evenly therein and to the upper cooking surface.
In order to thermostatically control the cooking surface of a utensil, electric skillets and crockpots have been developed. Typically, their heating element is embedded below the bottom surface of the utensil to adequately mask the heating element below or within the metal or ceramic mass of the utensil. This masking tends to avoid excessive local overheating. The disadvantages of such devices include increased cost, weight, and complexity. Further, these devices tend to gradually develop zones on the cooking surface which are effected by the heat and which change their cooking characteristics over time. In addition, the use of electricity for temperature maintenance is wasteful and costly. Finally, the electric cord and connector associated with these devices introduces the disadvantages of safety hazards, nuisance, functional obsolescence, and cleaning problems.
In order to better retain heat within cooking utensils after cooking, lids have traditionally been used. Nevertheless, utensils with lids tend to lose heat rapidly through their exterior surfaces by the processes of radiation, conduction and convection of heat. As a consequence, the most common solution is to continue to apply heat to the cooking utensil in some manner. This may include leaving the cooking utensil on the burner of a stove or range. Even with the burner adjusted to a minimum position, the prepared food may become dried out, burnt, or otherwise overcooked with time.
As an alternative, the cooking utensil may be placed on a food warmer such as is disclosed in Swiss Patent CH197804 which relates to a heat-retaining plate warmer including a hollow volume defined therein which contains a solid calorific material for retaining heat. The calorific material is solid at room temperature and retains heat over a relatively long time period when heated to a liquid state.
U.S. Pat. No. 5,125,391 issued to Srivastava et al. discloses a heat-retaining food service container with an outer shell and an expandable inner shell arrangement. Contained within the inner shell is a mass of heat-fusible material which expands when heated. The expandable inner shell thus expands or deforms as necessary to contain the material therein.
An insulated cooking utensil for retaining heat within the cooking utensil is disclosed in U.S. Pat. No. 3,777,094 issued to Peters, Jr. The utensil includes a metallic inner container for containing and cooking the food therein. Surrounding the metallic inner container are two layers of exterior insulating layers, which may be composed of high temperature plastic, ceramic, high temperature glass or the like. Each of the exterior insulating layers include a window opening defined along a bottom surface thereof to allow heat to be applied to the metallic inner container through high frequency magnetic induction fields.